Counter Rotating Propellers

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jedi

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According to this guy Paul LIPPS -- Elliptical Propeller you can get most of those advantages with a properly designed single prop.
Yes, but with a high speed engine (RPM) on a low speed airframe (MPH) you still need a gear reduction with all the associated problems. My proposal in post # 17 was to take advantage of the required gear reduction to gain aditional benifits that would otherwise not be available and at the same time eliminate some of the potential problems that the conventional gear reduction introduces. For example,the planitary set preserves the concentric relationship with the engine shaft; a requirement for some re-engine applications. The gear rerduction does not need to mount to the engine, it can be directly mounted to the output shaft same as a direct drive propeller. The airframe does not need to handle the greater torque reaction that would normally be created with the required gear reduction.


Furthermore, I have had experiance with propellers somewhat approaching the example given. They do work better than the conventional propeller, however, they are more difficult to manufacture and therefore more expensive and also, in my applicaton, were more suseptible to damage.
 
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DangerZone

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I have considered a planitary gear set where the motor drives the sun gear and the ring gear and planet set drive propellers in opposite directions and at different speeds. This would be a simple method to reduce tortional effects and obtain a significant gear reduction for high speed two cycle engines. Prop diameters would also be different with a large diameter low speed prop and a smaller diameter high speed prop. Tortional issues should also me minimized. I have considered using a three speed bicycle hub to do some experimenting but that is as far as the project has progresed.

Asside from the reduced tortional effects and simplified gear reduction, the proposed advantages would be 1. improved disk loading by providing higher velocity in the center of the propeller disk area; 2. Reduced noise for the same reasons as well as the lower tip speeds; 3. Better engine cooling particularly in pusher configurations such as paramotor back pack applications.

Is there any other interest in such a project.
A couple of years ago I talked to a guy who builds hovercrafts and they used to order coaxial shaft hubs from a Russian guy who calculated them pretty precisely. There were two variants, one with 8 and 6 blades and another with 12 and 9 blades. The reduction of speed and augmentation of torque had to be taken into consideration to get the desired efficiency increase, but most of these fans achieved 20% to 30% efficiency increase. I'm pretty sure he used a planetary gear setup, and these hovercrafts would accelerate to 100km/h in less than a second.

The DUAL rotating propeller(s) have a few % better efficiency, is noisier, more vibrations, more complicated, but pull straight.
This 'few' % better efficiency can be quite substantial if the job is well done. The counter rotation canceled by the opposite motion reduces propwash, the P-factor and torque effect so there are also less swirl losses. They are definitely more complicated and it is questionable if the increase in efficiency could justify the complex construction cost. Unless someone comes up with a very cheap design, homebuilts will hardly be able to afford the luxury of counter rotating props/fans. Airliners might, but that's a different story.
 

deskpilot

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....... Only the Supermarine Spitifire was made in significant numbers with Rolls Royce Griffin engines and contra-rotating props.
You're forgetting the Wessex Wyvern which used Griffins prior to going turbine powered.

3970.jpg Only surviving type with Griffin engine.

Westland Wyvern.jpg Turbo S4, I believe I'm correct in saying this was the fastest prop aircraft ever built. None survive today. Was used by the royal Navy in the Suez conflict.
 

Jan Carlsson

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This 'few' % better efficiency can be quite substantial if the job is well done. The counter rotation canceled by the opposite motion reduces propwash, the P-factor and torque effect so there are also less swirl losses. They are definitely more complicated and it is questionable if the increase in efficiency could justify the complex construction cost. Unless someone comes up with a very cheap design, homebuilts will hardly be able to afford the luxury of counter rotating props/fans. Airliners might, but that's a different story.
But remember that the extra gear set, bearings and weight also take some power, I designed 2-3 (pair) of dual props, I guess the lack of torque and prop factors, is just as big gain as the theoretical gain in efficiency.

As the designer of the Volvo duo-prop and IPS, drive said, "the only gain was the smaller gear house" (= less drag)
 

Dana

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I know I tend to use this expression fairly frequently, but I think this is another "solution in search of a problem." It made sense in the closing days of propeller driven fighters, where the the power available was outstripping the ability of any single propeller of reasonable diameter to handle it, and also to counteract the torque and P-factor involved. But if you aren't dealing with that amount of power, it's not worth the complexity of the gearbox.

There have been some in recent decades, though. The early ParaPlane powered parachutes had coaxial counterrotating props belt driven from two 15HP Solo engines, and there was at least one PPG (powered paraglider) driving CR props through a gearbox and one engine. In both of these cases it was due to to the [real or perceived] inability of the parawing to handle the torque, and in the case of the ParaPlane, the unavailability of a single engine with the power to weight ratio of the twin Solos.

Dana
 

DangerZone

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Zero to over 60 MPH in one second????
Yeah, there were some videos of hovercraft racing, amazing static thrust. They don't go much faster than 130km/ but the acceleration is great.

But remember that the extra gear set, bearings and weight also take some power, I designed 2-3 (pair) of dual props, I guess the lack of torque and prop factors, is just as big gain as the theoretical gain in efficiency.

As the designer of the Volvo duo-prop and IPS, drive said, "the only gain was the smaller gear house" (= less drag)
Indeed, any improved efficieny is ok for the big birds but not much hapiness for us homebuilders where the complexity is the opposite of what we seek...
 

SkyClimber

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It is true that the Cotra-rotation application has been used on high powered aircraft, mainly for the reasons stated by Riggerrob. However, there is a trend to use them on the lower powered vehicles such ultralights, powered Trikes and powered parachutes as well, vehicles which one would think would not tolerate a "weight penalty".
What it all comes down to is cost/benefit ratio.
The big guys used them because they had plenty of power and wanted to negate adverse torque and also squeeze every bit out of a propeller driven system. Witness the Russian Tupolev, the fastest prop driven aircraft in the world and racers and fighters which were already heavy and don't mind some added weight in order to maximize performance.
The little guys are running fast rpm engines and moving relatively slowly and therefore often need prop speed reduction. Adding a Contra-rotation takes little added weight, reduces vibration and increases the efficiency of the props, again squeezing every bit of power out what they have.
I know I sound like CR props are the best thing since sliced bread...but they are not for eveyone. For most airplanes out there it is best to keep it simple and run a single rotation. But they can be invaluable in certain situations as I have described, expecially those discussed in this forum such as flying platfoms, and in other applications which need to negate adverse torque and precesion forces such as in VTOLS in slow flight/hover. Even Airboats are running CR props for the same reasons!
 

DangerZone

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It is true that the Cotra-rotation application has been used on high powered aircraft, mainly for the reasons stated by Riggerrob. However, there is a trend to use them on the lower powered vehicles such ultralights, powered Trikes and powered parachutes as well, vehicles which one would think would not tolerate a "weight penalty".
What it all comes down to is cost/benefit ratio.
The big guys used them because they had plenty of power and wanted to negate adverse torque and also squeeze every bit out of a propeller driven system. Witness the Russian Tupolev, the fastest prop driven aircraft in the world and racers and fighters which were already heavy and don't mind some added weight in order to maximize performance.
The little guys are running fast rpm engines and moving relatively slowly and therefore often need prop speed reduction. Adding a Contra-rotation takes little added weight, reduces vibration and increases the efficiency of the props, again squeezing every bit of power out what they have.
I know I sound like CR props are the best thing since sliced bread...but they are not for eveyone. For most airplanes out there it is best to keep it simple and run a single rotation. But they can be invaluable in certain situations as I have described, expecially those discussed in this forum such as flying platfoms, and in other applications which need to negate adverse torque and precesion forces such as in VTOLS in slow flight/hover. Even Airboats are running CR props for the same reasons!
Any resources on the comparison of American, European and Russian CR gears/drives/hubs available online..? I've seen a coaxial CR fan system from a Russian guy and a couple of central European solutions and they differ quite a bit.
 
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